This invention relates to a method for the recovery of lanthanide and actinide values from aqueous acid solutions. More specifically, the invention relates to a method for the recovery and separation of multivalent lanthanide and actinide values from aqueous acid waste solutions containing these and other values.
The disposition of the waste which results from the reprocessing of irradiated nuclear power reactor fuel elements is one of the major problems facing the nuclear power industry today. One approach is to solidify the liquid wastes as they come from the reprocessing facility into a stable solid material which can be stored in the earth for a period of time sufficient for the radiation to decay to safe levels. However, the storage times required to achieve safe levels of radioactivity are on the order of one million years. This is far longer than the geological stability of the earth can be expected to be maintained. One solution is to remove the extremely long-lived radioactive components, such as the actinides, i.e. the americium, curium, plutonium and neptunium from the waste solution so that the remaining radioactive elements, representing the bulk of the radioactive waste, need only be stored for up to 1000 years before the radioactivity decays to background levels. This time period is within the realm of determining geological integrity. The lanthanides and actinides thus recovered from the waste can then be reprocessed and recycled to provide additional fuel for nuclear reactors and for isotopic power sources.
Another approach is to minimize the volume of radioactive waste that must be buried by first removing the actinides, particularly plutonium and americium, and then removing the major heat producing isotopes .sup.90 Sr and .sup.137 Cs. The remaining waste is not sufficiently radioactive to require placement in a deep geological repository. .sup.90 Sr and .sup.137 Cs have commercial applications. Only the actinide fraction would require placement in a deep geological repository. The volume of the TRU fraction (neptunium, plutonium, and americium) together with lanthanide fission products is approximately 1% of the total high level radioactive waste generated in a plutonium production plant.
The problem with recovering the lanthanides and actinides from large volumes of acidic high-level radioactive waste solutions is to find methods, using extractants, which will do so effectively and economically, since the lanthanides and actinides are present in several valence states and are difficult to recover together.
One group of extractants which has been considered for the extraction of lanthanide and actinide values from aqueous acid waste solutions are the bidentate organophosphorous compounds. These compounds are described in U.S. Pat. No. 3,243,254 which issued Mar. 29, 1966 to Thomas Siddall III and is assigned to the common assignee. These extractants, include di-n-hexyl N,N-dibutylcarbamylmethylenephosphonate, di-n-octyl N,N-diethylcarbamylphosphonate, dibutyl N,N-diethylcarbamylphosphonate, and dihexyl N,N-diethylcarbamylmethylene phosphonate. These extractants have generally proven satisfactory, although distribution ratios for some lanthanides and actinides are small, so that separation from other values found in the reprocessing solution is not always satisfactory. Furthermore, acid concentration ranges of the feed solutions are critical to extractability and the extractants are susceptible to radiolytic and hydrolytic damage which reduce their effectiveness.